1. Field of the Invention
The present invention relates to an opto-electric hybrid board including an electric circuit board and an optical waveguide which are stacked together.
2. Description of the Related Art
With the increase in the amount of transmission information, optical interconnection in addition to electrical interconnection has been used in recent electronic devices and the like. As an example of such a technique, an opto-electric hybrid board as shown in FIG. 7 has been proposed (as disclosed in International Publication No. WO 2010/058476, for example). This opto-electric hybrid board includes: an electric circuit board E0 including an insulative flexible substrate (insulative layer) 51, electrical interconnect lines 52 formed on the front surface of the flexible substrate 51, and an insulative coverlay (interconnect line cover layer) 53 formed on the front surface of the flexible substrate 51 and for covering and protecting the electrical interconnect lines 52; and an optical waveguide (an optical interconnect line) W0 (including an under cladding layer 56, cores 57 and an over cladding layer 58) made of epoxy resin and the like and stacked on the back surface (a surface opposite from the surface with the electrical interconnect lines 52 formed thereon) of the flexible substrate 51 of the electric circuit board E0. Both of the electric circuit board E0 and the optical waveguide W0 in the opto-electric hybrid board are thin and accordingly flexible. To meet the requirement for the size reduction of recent electronic devices and the like, the opto-electric hybrid board has been used in a small space when in a bent position, and used in a movable section such as a hinge.
In the aforementioned opto-electric hybrid board, the coverlay 53 which covers and protects the electrical interconnect lines 52 and the optical waveguide W0 are in general disposed in stacked relation (in vertically stacked relation as seen in FIG. 7). Thus, the thickness of the aforementioned opto-electric hybrid board is the sum of the thickness of the electric circuit board E0 and the thickness of the optical waveguide W0. When such an opto-electric hybrid board is bent, stresses are applied to the bent portion of the opto-electric hybrid board due to the thickness thereof. As a result, there is a danger that a break or a crack occurs in the bent portion to impair the achievement of proper information transmission.